The term “frailty” is used to represent an unobservable random effect shared by subjects with similar risks. Frailty models have been introduced into the statistical literature to account for the existence of unmeasured attributes, such as genotypes that introduce heterogeneity into a study population. Including inference results, applications, and references, this book presents the basic concepts of frailty models in survival analysis. The author also addresses advanced topics in frailty models, including estimation procedures, tests for frailty, and additive frailty models. The text covers frailty models based on Levy processes as well as correlated frailty models.